Film formation on valve metals



Nov. 14, 1933. E. w. ENGLE 1,935,395

FILM FORMATION ON VALVE METALS Filed Aug. 29, 1927 2 Sheets-Sheet l Me Qaihlsw W Patented Nov. 14, 1933 UNITED STATES PATENT OFFILCE Fansteel Products Company,

Inc., North Chicago, 11]., a corporation of New York Application August 29, 1927. Serial No. 216,123

' Claims. (01. 175315) My invention pertains to the formation of a resistant film on valve metals such as aluminum, tantalum, etc., and it relates more particularly to the formation of a film on aluminum plates to be used in an electrolytic condenser of the type wherein the electrolyte is dissolved in a glyceringelatin gel or in substantially dry glycerin.

One of the most serious disadvantages of the electrolytic condenser is the breaking down or dissolving of the film while the condenser is standing idle.

The object of this invention is to provide a means for forming a film on a condenser electrode which will be permanent and which will not be aiiected by periods of idleness.

Another object of my invention is to provide a means for removing impurities from the electrodes preparatory to forming the film.

A further object of this invention is to provide a film through which leakage is reduced to a minimum.

A further object of my invention is to provide a filmed electrode especially adapted for use in condensers containing an electrolyte dissolved in glycerin.

A further object of my invention is to provide an improved means for suspending condenser plates in the forming bath.

Another object is to provide an improved formv ing process.

I I' Other objects will be apparent as the detailed description of my invention proceeds.

A preferred embodiment of my invention is shown in the accompanying drawings, in which Fig. 1 shows diagrammatically the forming bath and electrical connections for supplying direct and alternating current;

Fig. 2 shows a similar bath wherein the electrolytei's dissolved in glycerin;

Fig.3 is a perspective of the electrode holder;

Fig. 4 is a perspective of a single condenser plate or electrode;

Fig. 5 shows diagrammatically a non-aqueous electrolytic condenser using said plates; and

Fig. 6 is a flow sheet illustrating my improved process.

The first step in my process, as shown in the flow sheet, is the removal of impurities from the electrode prior to its treatment in the forming bath. It is very important that all grease and other organic or inorganic impurities be removed in order that the film may be continuous and unimpaired. This is accomplished by treating the plates with a hot saturated solution of sodium carbonate. This treatment may be accompanied by scrubbing, if necessary, and in the preferred embodiment should last about five minutes. While sodium carbonate is the preferred reagent in the primary cleaning step, it is understood that any other solution may be used which will remove the grease and impurities and which will not attack the aluminum.

The plates are then thoroughly rinsed, after which they are transferred to an acid solution for-dissolving off any metal impurities. This 50- lution is preferably nitric acid (about 25%) although any acid may be used which does not affect aluminum and which will dissolve other metal impurities.

The plates are then thoroughly washed and rinsed with water, after which they are inserted in the holder shown in Fig. 3.

The plates as shown in Fig. 4 are preferably stamped aluminum sheets 10 provided with corrugations 11 for increasing the exposed surface area. Each plate is provided with a terminal projection 12 as shown in Fig. 4. The plate holder is preferably an aluminum strip 13 provided with a series of slots 14 adapted to frictionally engage the projections 12.. By this means a large number of electrodes may be supported in a suitable electrolyte 15, the holder also serving as a conductor as shown in Figs. 1 and 2.

The plates or electrodes which are now thoroughly cleaned are immersed in the electrolyte, preferably a one or two percent solution of sodium bicarbonate in water to which a little glycerin may be added to prevent corrosion at the terminals. Other electrolytes may be used but I have found that sodium bicarbonate gives a more uniform and permanent film than other electrolytes. The sodium bicarbonate solution is contained in a suitable receptacle 16 which is preferably of iron or other conductive material so that it may be used as one electrode in the forming circuit. 95

Wires 17 and 18 connect the holder 13 and receptacle 16, respectively, to the blades of a double throw switch 19. In one position this switch closes a circuit to a D. C. source through conductors 20 and in another position it closes a 100 circuit to an A. C. source through conductors 21,

An A. C. potential is first applied and is carried to about 325 volts. The forming current may be regulated by suitable choke coils and maintained at any desired value, depending upon 105 the area of the plates used, but the value of the current should preferably be kept sufiiciently low at all times so that the plates will not be overheated and the temperature of the electrolyte will not exceed about 60 C. This A. C. form- 110 ing treatment usually requires about six minutes.

The switch 19 is now thrown to the position shown in Fig. 1 and the electrodes are subjected to a D. C. potential of about 280 volts. This treatment requires about two hours and should be continued until leakage current is substantially zero. The holder is then removed from the first forming tank and the plates are rinsed and thoroughly dried.

The next step is the treatment in glycerin. The solution is prepared by dissolving about one ounce of sodium bicarbonate in a liter of glycerin. It is understood that here again various electrolytes may be substituted for sodium bicarbonate without departing from my invention, but I have found that sodium bicarbonate gives the best results.

A D. C. potential of about 250 volts is then impressed on the plates and is maintained till the gassing ceases, the step usually requiring about six hours. The tank for this treatment is substantially the same as in Fig. 1 and comprises a receptacle 16' containing a glycerin solution 15. The holder 13 and receptacle 16' are connected to the D. C. source by suitable conductors 22.

After the fine streams of bubbles have ceased to flow from points on the surface of the plates, they are removed, drained, and with the film of glycerin still adhering to their surfaces they are introduced into the non-aqueous electrolytic condenser. Such a condenser is diagrammatically shown in Fig. 5 wherein a suitable container 23 of glass, hard rubber, metal, etc., is filled with a solution 24 of glycerin and sodium bicarbonate to completely immerse the formed plates 10.

In order to prevent the absorption of water by my glycerin solution, I provide a layer of oil 25. This seal permits the escape of any gas bubbles that may be formed due to leakage currents and at the same time prevents evaporation of the solution and water absorption.

Condenser plates which are formed in aqueous solutions and then introduced into glycerin electrolytes cause trouble due to the leakage current. By including the step of film forming in the glycerin solution this difliculty is avoided. The thin layer of glycerin which adheres to the plate on its removal from the glycerin forming bath also serves to make better contact with the electrolyte in the condenser cell; that is, there is less likelihood of air pockets being formed, etc.

The filmed electrodes prepared by the above process are uniform and substantially permanent and they do not require reforming after a period of idleness.

While I have described in detail specific embodiments of my invention, it is understood that I am not limited to the details set forth except as defined by the following claims. ,The expression substantially dry" as used in the appended claims does not mean that no trace of water is present, but that the electrolyte is essentially a solution of an ionogen in glycerine as distinguished from an aqueous solution to which glycerin has been added.

I claim:

1. lhe method of forming a film on a valve metal plate which comprises subjecting it to A. C. in an aqueous electrolyte, then to D. C. in an aqueous electrolyte, and finally to D. C. in a substantially dry glycerin solution.

2. The process of forming an aluminum plate which comprises subjecting it to an A. C. voltage in a dilute solution of sodiumbicarbonate, subjecting it to a D. C. potential in the same solution, and subsequently subjecting it to a D. C. potential in a solution of sodium bicarbonate in glycerin.

3. The process of forming an aluminum plate which comprises subjecting it to an A. C. potential of about 325 volts in a dilute solution of sodium bicarbonate, subjecting it to a D. C. potential of about 280 volts in the same solution, and subsequently subjecting it to a D. C. potential of about 250 volts in a glycerin solution of sodium bicarbonate till bubbles cease to form at the surface of the film.

4. The combination of claim 3 wherein the first step requires about 6 minutes, the second step about 2 hours, and the third step about 6 hours.

5. The method of forming a film on aluminum plates which comprises treating the plate with solutions for removing organic and inorganic impurities, subjecting said plate to an A. C. potential in an aqueous electrolyte, subjecting it to a D. C. potential in the same electrolyte, and finally subjecting it to a D. C. potential in a glycerin solution.

6. A method of forming a film on a valve metal which comprises electrically forming the metal in an aqueous solution of an electrolyte, and completing the film formation in an electrolyte incorporated in glycerin.

EDGAR W. ENGLE. 

